1. Field of the Invention
The present invention relates to condensate demineralization using ion-exchange resin in a multiple-bed regime and more particularly relates to a multiple bed-type condensate demineralization method and apparatus for the method that are used in steam power plants and that can reduce the frequency of chemical regeneration.
2. Description of the Related Art
The condensate demineralizer installed in the secondary system of a pressurized water reactor (PWR) plant carries out the demineralization of ionic impurities (e.g., Na, Cl, SO4) in the condensate, thereby reducing the ionic impurities that will be carried into the steam generator through the feed water system to very low levels with the goal of stopping stress corrosion cracking, which is at risk of occurrence when the ionic impurities become concentrated, in the heat transfer pipework of the steam generator. In addition, in order to inhibit the inflow of crud (iron oxide) to the steam generator, an all volatile treatment (AVT) is carried out in the secondary system of PWR plants, in which, for example, ammonia and hydrazine are added to the condensate and the pH is controlled into the basic region.
Thus, in order to secure a very pure water quality, operations are carried out at the condensate demineralizer of PWR plants using a demineralizer having a mixed bed of H-form cation exchange resin and OH-form anion exchange resin (hereinbelow, simply referred to as an anion resin); however, the ammonia and hydrazine added to the condensate impose a large load on the cation exchange resin (hereinbelow, simply referred to as a cation resin) making it necessary to carry out chemical regeneration frequently due to the rapidity of ion breakthrough at the cation resin. This entails several problems for the PWR plant that must be addressed, such as economic problems such as an increase in the frequency at which the ion-exchange resin must be changed out, the consumption of large amounts of regenerant, pure water, steam, and air, and an increase in chemical waste solutions requiring treatment, as well as an increased burden on the operating personnel, the occurrence of oxidative deterioration of the cation resin during chemical regeneration, and a reduction in the reaction rate of the anion resin (hereinbelow, simply referred to as anion resin).
In addition, as the elapsed time in service by the ion-exchange resin grows, oxidative deterioration of the cation resin gradually progresses due to the frequent execution of chemical regeneration and the amount of eluted organic impurities undergoes a gradual increase. As a consequence, the sulfate ion produced by decomposition of the organics gradually undergoes an increase in concentration, while the surface of the anion resin is contaminated by the eluted organic impurities and the quality of the treated water declines due to a reduction in the reaction rate.
The ammonia operation of the condensate demineralizer, which enables a reduction in the frequency of chemical regeneration, can be considered as an approach that solves this problem, but the heretofore known ammonia operation provides a treated water quality inferior to that of H,OH operation and thus cannot be applied as such to PWR plants.
Methods that incorporate the use of an intermediate resin as in Japanese Patent Application Laid-open No. Hei 8-117615, the use of caustic soda for resin separation as in Japanese Patent Application Laid-open No. Sho 55-94650, and resin transfer as in Japanese Patent Application Laid-open No. Hei 4-100587 have been put forward as methods for reducing the cross contamination ratio; however, substantial reductions in the cross contamination ratio are a highly problematic proposition as long as the cation resin and anion resin are used in a mixed bed. In addition, Japanese Patent Application Laid-open No. Hei 11-352283 discloses a method that uses a highly crosslinked strong acid gel-type cation resin to halt the decline in water quality caused by the development of oxidative deterioration; however, when this is implemented in a mixed-bed regime, cross contamination occurs as cited above, making a reduction in water quality unavoidable.